1. Field of the Invention
The present field of the invention relates to a process and network in which an executive monitor is connected within a microwave landing system ("MLS") to evaluate whether or not an internally-generated out-of-tolerance signal activates an alarm system. If the alarm system is activated, then the proper fault monitoring function of the executive monitor is verified.
2. Description of the Prior Art
An instrument landing system ("ILS") has served as the prior art approach and landing aid for aircraft for many years. The ILS, however, has a number of basic limitations, such as being site critical and expensive to install, being sensitive to extraneous reflections, having a limited number of channels, lacking the flexibility required for aircraft operations, and producing erroneous information in rough terrain and mountainous regions. As a result of these limitations, an MLS has been proposed as a standard ILS replacement for world-wide implementation since it can reduce or eliminate these basic limitations.
The MLS consists of various antenna stations adjacent to a runway which transmit wave energy information to approaching aircraft enabling said aircraft to calculate the following data to safely land on an airport runway: azimuth from an AZ station, elevation from an EL station, range from a precision distance measuring equipment (DME/P) station, and back azimuth from a BAZ station. The AZ station provides an aircraft with heading or approach guidance to runways or helo pads at an airport. The EL station provides for a wide selection of glide slope angles needed by a pilot to land his plane safely on a runway. The DME/P station provides for continuous range information needed by a pilot to ascertain the distance between his aircraft and the airport runway on which he is landing. The BAZ station is similar to the AZ station and is intended to supply guidance to a pilot for missed approaches to and departures from an airport.
More specifically, the AZ station includes an antenna which generates a narrow, vertical, fan-shaped beam which sweeps to and fro across the area to be covered by the AZ station. Before the start of a scan a test pulse is transmitted, then the "to" scan starts. At the end of the scan, there is a pause before the "fro" scan starts. A second test pulse marks the end of the scanning cycle. The aircraft receives a "to" pulse and a "fro" pulse. The time difference between pulses is then measured by the aircraft and gives the angular location of the aircraft relative to the AZ station. The EL station also includes an antenna which generates a narrow horizontal fan-shaped beam which sweeps up and down through the area to be covered at the airport. The time difference between receipt of the up and down pulses is used by the aircraft to determine the elevation angle of the aircraft relative to the EL station and thus its displacement from the glide path angle selected by the pilot to land his aircraft on a runway. The elevation scan cycle requires much less time than the azimuth scan cycle. The elevation scan cycle is normally repeated 39 times per second as compared with 13 times per second for the azimuth cycle. The BAZ station includes an antenna which generates a narrow, fan-shaped, vertical beam which sweeps to and fro horizontally through the area to be covered at the airport. The same angular measurement principle used for determining the approach AZ angle is used for determining the BAZ angle. The DME/P station includes an antenna which transmits wave energy travelling at a known rate. By calculating the time the wave energy travels from the antenna to the aircraft and knowing the rate at which the wave energy travels, the distance or range between aircraft and the airport station can be calculated. The above information is calculated by the approaching aircraft as a direct result of the meaningful information transmitted by the antenna stations included within a given MLS at an airport.
The MLS is capable of operating on any one of 200 channels in the microwave frequency band. The present microwave frequencies in use are between 5043 and 5090.7 megahertz. The AZ, BAZ and EL stations all transmit on the microwave frequency. The DME/P station transmits a paired frequency in L-Band. The MLS signal format has the potential to transmit signals from the above-mentioned various stations in any desired order to approaching aircraft. A preamble or data word is transmitted by each station to approaching aircraft prior to the main wave energy being transmitted in order to inform the approaching aircraft of which function (AZ, EL, BAZ, DME/P) will be transmitted next. As soon as the aircraft decodes the message it waits for the wave energy to be received in order to perform the desired calculation. Then, the aircraft awaits the next preamble to ascertain the identity of the next transmitted function. As can readily been seen, the MLS has numerous advantages over the ILS.
All MLS installations transmit the following basic data to approaching aircraft: facility (landing runway) identification; azimuth threshold distance, coverage and off-set (distance from AZ antenna to fixed spot on center line of runway); beam widths (AZ, EL); DME/P distance, off-set and channel (distance between station and runway); and elevation height, off-set and distance from threshold. Most of this information is needed by the equipment aboard the approaching aircraft to make the necessary computations for an approach to the airport. Any malfunction in the MLS equipment will cause the approaching aircraft to make faulty calculations and rely upon erroneous data. For this reason, it is absolutely essential to continuously maintain the MLS system and to verify that the MLS system stations are transmitting accurate information.
The MLS was the first system designed to utilize a maintenance program. The advantages of such a maintenance program include a reduction in the time spent in travel maintaining the system and a reduction in the maintenance and record-keeping for the system, which in turn allows more effective use of a smaller number of maintenance personnel operating from a smaller number of maintenance bases. Overall, such a maintenance program is economical, reliable, and efficient.
Each MLS station is supported by an executive monitor and a maintenance field monitor, both of which are tools implementing the maintenance program. The executive monitor samples the information being transmitted by each antenna station to approaching aircraft. In other words, the executive monitor evaluates the same information that the antenna is transmitting to approaching aircraft to ensure that the information being sent to the aircraft is reliable. For example, the executive monitor checks the accuracy of the angle code throughout the antenna coverage and thus can detect when a given sample of the angle code is beyond a predetermined limit. Examples of such transmitted parameters which are checked by the executive monitor to ascertain whether or not they are out-of-tolerance are (a) scanning beam mean angle error, (b) function preamble, (c) effective radiated power (whether it be for the function preamble, the EL and BAZ scanning beams, clearance pulses, or an out of coverage indicator), (d) timing error in signal format, (e) synchronization error in time division multiplexing, (f) digital phase shift keying ("DPSK") data transmission, (g) interstation synchronization, (h) array integrity parameters (such as dynamic sidelodes, channel failures, frequency channels, etc.), and (i) clearance angle. If any of these transmitted parameters a-j are out-of-tolerance, then the executive monitor automatically initiates an alarm, the station is shut down, and the approaching aircraft does not receive information from that station. The AZ, EL, BAZ, and DME/P antenna stations each contain an executive monitor. If the EL or BAZ station is shut down, the other stations are still operable and transmit information to the approaching aircraft. If the AZ station is shut down, however, all stations are disabled and do not transmit information to approaching aircraft. Since the executive monitor initiates an alarm and shuts down a station or the system when an out-of-tolerance parameter is detected over a predetermined period of time, it is necessary to verify that the fault monitoring function of the executive monitor is operating properly and will provide such an alarm when such an out-of-tolerance parameter is detected over that predetermined period of time. Otherwise, an approaching aircraft may be erroneously relying on MLS supplied information which should have generated an alarm and shut down the station or the MLS without transmitting information to approaching aircraft.